The present invention relates to the manufacture of beads of acrylic (co)polymers, in particular of beads suitable for the applications as coating.
It is well known that for certain coatings, for instance in certain dental applications, beads having a granulometry from 0.03 to 0.100 mm are required.
In particular the present invention relates to the manufacture of beads of acrylic (co)polymers, obtained by processes in aqueous suspension in the presence of suspending agents, utilizable as coating.
It is known that with the processes of polymerization in suspension, beads of polymers having an average diameter between 0.1-1 mm are obtained.
The granulometric size distribution is very large and generally one proceeds to separation by screening if a certain kind of average size is desired. However the drawback from the industrial point of view is that for particles for coating with the indicated sizes, the screening is not very effective and is complex and besides there is the further drawback that the yields are very low.
It is not therefore assumable from the industrial point of view a production by using these processes known in the art.
The polymerization in suspension is a reaction type which is performed in a system in which the monomer is suspended in the form of small drops in a continuous phase and polymerized using an initiator of radical type soluble in the monomer. The continuous phase is generally water.
The ratio between continuous phase (water) and discontinuous phase (monomer) is generally comprised between 1:1 and 3:1.
In the practical embodiment of this type of process it is necessary the employment of suspending stabilizers hindering the coalescence of the small drops of monomer in the most advanced steps of the polymerization.
As suspending stabilizers are used, in the most usual technique, hydrosoluble macromolecular compounds with affinity towards the monomer which, placing themselves at the interface between organic phase and aqueous phase, form a protective film hindering agglomeration of the particles.
At the end of the polymerization the suspending agent is removed from the surface of the polymer particles by washing with water.
The suspending agent is a key factor as its characteristics conditions the performances of the whole process both from the point of view of the quality of the final product and from the point of view of costs.
Polymerization processes in aqueous suspension of acrylic monomers are also known, in particular the one described in European patent application 457,356, which corresponds to U.S. Pat. No. 5,344,901 which is herein fully incorporated by reference, wherein particular polymers selected from the homopolymers of compounds of formula ##STR2##
are used as suspending and stabilizing agent of the aqueous suspension,
wherein: R.sub.1 =H, CH.sub.3 ; R.sub.2 and R.sub.3, equal or different, are H, alkyls C.sub.1 -C.sub.8, and M=an alkaline or alkaline-earth metal, or copolymers of said compounds with acrylic monomers.
By using said suspending agents various advantages are obtained, such as:
high stability of the aqueous suspension, also operating with ratios water/acrylic monomers near unity PA1 waste waters having a very low content in residual polymer PA1 obtainment of acrylic polymers having high optical purity. PA1 a) the homopolymers of a compound of formula: ##STR3## PA1 wherein: R.sub.1 is H or CH.sub.3 ; R.sub.2 and R.sub.3, equal or different, are H or alkyls C.sub.1 -C.sub.8, optionally branched when possible; M is an alkaline or alkaline-earth metal or ammonium and A is NH, O or NCH.sub.3, PA1 b) the copolymers of said compound of formula I with 40% by weight at most of acrylic monomers, PA1 drastic reduction of the amount of mother waters to be removed PA1 drastic reduction of the specific consumption of suspending agent without, however, modifying the good mechanical and optical properties of the polymers so obtained. PA1 a) homopolymers of a compound of formula: ##STR4## PA1 wherein: R.sub.1 is H or CH.sub.3 ; R.sub.2 and R.sub.3, equal or different, are H or alkyls C.sub.1 -C.sub.8, optionally branched when possible; M is an alkaline or alkaline-earth metal or ammonium and A is NH, O or NCH.sub.3, PA1 b) the copolymers of said compound of formula I with 40% by weight at most of acrylic monomers, PA1 a) homopolymers of a compound of formula: ##STR5## PA1 wherein: R.sub.1 is H or CH.sub.3 ; R.sub.2 and R.sub.3, equal or different, are H or alkyls C.sub.1 -C.sub.8, optionally branched when possible; M is an alkaline or alkaline-earth metal or ammonium and A is NH, O or NCH.sub.3, PA1 b) the copolymers of said compound of formula I with 40% by weight at most of acrylic monomers,
However, according to the examples of EP Patent 457,356 the obtained beads have average sizes from 0.2 to 0.3 mm. It is known that in the industrial plants, which are large-sized, there exist some bonds to the stirring, wherefore it is not industrially suggestable to act on this parameter to obtain fine particles for coating.
It was felt the need to have at one's disposal an industrial process with high yields making available beads of average sizes from 0.03 to 0.100 mm to be used as coating and at the same time capable of utilizing the advantages indicated for the suspending agent cited above.
It has now been unexpectedly and surprisingly found that this is possible using the process described hereunder.